Floating Drilling, Production, Storage and Offloading (FDPSO) is subject to combined loads including wind, wave and current loads, which affect the fatigue damage of FDPSO mooring system respectively. In this paper, the effects of wind and current loads to the fatigue damage of FDPSO mooring system are presented by counting damages under wave-wind, wave-current and wave-wind-current cases. Firstly, according to the time domain coupled analysis method, the numerical reconstruction and extrapolation are carried out by using the model test results and ANSYS AQWA software, and tension time- histories of mooring lines under the three cases are obtained. Secondly, the fatigue load spectra under short-term sea states are counted out by Rain-flow counting method. Then the fatigue damage of mooring line is calculated according to T-N curves and Miner linear cumulative damage theory.
FDPSO is placed in oilfields for a long time and several mooring lines are employed to provide the principal resistance to displacements induced by the environmental loading. Providing the FDPSO mooring systems are subjected to alternating loads at sea, its fatigue strength must be considered in the design and production process. Several studies have been conducted on the fatigue strength of mooring lines by different methods. In time domain, Luo (1990) studied the non-Gaussian characteristic of fatigue load spectrum. Lassen (1996) used Gaussian distribution model to fit the relationship between fatigue damage and service time of steel chain. Vazquez-Hernandez (1998) predicted the fatigue life of a floating structure mooring line by different statistical methods, Dirlik, Rain-flow, Narrow-band and Wideband included. Mathisen (1999) pointed out that the wave-frequency load causes more fatigue damage than the low-frequency part. Gao (2006) investigated the catenary mooring fatigue damage of a semisubmersible under the combined wave-frequency load and low- frequency load. Taking the correction into consideration, Qiao (2014) compared the fatigue life of two types of catenary. This paper points out the effects of wind and current loads to the fatigue damage of FDPSO mooring line which is multi-section catenary.